The use of RFID tags or cards to identify a person or an object is well known. In general, such tags, when excited, produce a magnetic field, or in some cases an electric field, of a first frequency which is modulated with an identifying code. The tag may be either an active tag, i.e., a tag which has a self contained power supply or, as is more usually the case, may be a passive tag that requires external excitation when it is to be read or disposed within the detection volume of a reader, for example, a door way or portal. In general, for portal type devices, passive type devices are utilized wherein a transmitting antenna for transmitting an exciting frequency signal for the tag is positioned at the portal and generally adjacent an antenna for receiving the modulated magnetic field, or electromagnetic field, produced by the excited tag in order to identify the tag, and consequently the user or object attached thereto.
A problem occurs in the detection of RFID tags in large areas or volumes wherein the horizontal and vertical dimensions of the detection volume in which the RFID tag is to be read exceeds the read range or detection geometries of a single reader. Moreover, detection problems occur if the tag to be read is not restricted to a specific orientation as it passes through the portal (detection volume), for example, if the tag is carried by a person passing through the portal or is attached to a randomly oriented object, such as a parcel or a package disposed on a conveyor. In particular, the detection volume of a single reader may not span the required volume with sufficient field strength to allow the tag to be read in the corners of the volume that are located at a distance from the surface of the reader.
In order to attempt to solve this problem, it is known to use a single reader with a single planar loop antenna located around the periphery of the portal area at a forward or rear edge of the portal. However, the detection volume of a single radio frequency reader, particularly of the proximity type for detecting a magnetic field, does not span the required portal volume with sufficient field strength to allow reading of the tag at all positions. Moreover, the loop geometry may create an area wherein under certain orientations of the tag, for example, parallel or perpendicular to the floor, the field strength of the single reader may be insufficient to fully power the tag (in the case of a passive tag) thus tending to produce a weakened identification signal. Consequently, the problem exists with this arrangement of providing sufficient field strength from the tag to permit detection within the detection range of the single reader.
To attempt to overcome this problem, it is also known to use multiple reader assemblies placed in an array with each other about the portal or detection volume, with the analog signals detected by the respective readers being tied together at a single RF detection and decoding device. It is known to provide these readers either in the same plane or facing each other across the portal in order to improve the field strength at the tag in the area of detection. However, the problem still arises with regard to the detection of the RFID signal, particularly when two detected signals are 180.degree. out of phase as can result either due to a connection of the antennas out of phase or due to the orientation of the tag relative to the planes of the antennas. For example, if a pair of antennas are disposed on opposite sides of the portal, a tag having its surface oriented parallel to the two receiving antenna coils would produce out of phase signals in the individual antenna coils while a tag having its surface oriented perpendicular to the two antenna coils would produce in phase signals in the individual antenna coils.
Accordingly, there is a need for an improved antenna array that detects a radio frequency identification (RFID) tag signal generated by a tag during passage of the tag through a detection volume. Further, such an antenna array that provided improved signal detection for use by a single receiver would be an improvement over the prior art.